1. Field of the Invention
The present invention relates to a side light type spread illuminating apparatus, and particularly to a spread illuminating apparatus for use as a lighting means of a liquid crystal display device.
2. Description of the Related Art
A liquid crystal display (hereinafter referred to as “LCD”) device is extensively used as a display means in recent electronic devices. Since the LCD device does not emit light by itself, a lighting means is required in order to ensure visibility for use at night or in the dark. A spread illuminating apparatus has been used as the lighting means, and side light spread illuminating apparatus, which is one type of spread illuminating apparatus, is prevailing.
The side light type spread illuminating apparatus basically includes a light conductor plate having light transmitting property, and a line light source or one or more point light sources disposed at a side of the light conductor plate. The side light type spread illuminating apparatus is increasingly applied in a small electronic device, such as a mobile information terminal, and therefore preferably incorporates a point light source which enables simplification of a driving circuit.
FIG. 5 shows a conventional spread illuminating apparatus 10 incorporating a point light source. The spread illuminating apparatus 10 includes a light conductor plate 12, and a light emitting diode (LED) 14 as a point light source disposed close to a light inlet surface 12a of the light conductor plate 12, wherein the number of the LED 14 is determined according to the brightness required for the apparatus 10 and the area of the light conductor plate 12. The spread illuminating apparatus 10 further includes a light diffuser sheet 16 disposed on a light outlet surface 12b of the light conductor plate 12, and a prism sheet 18 disposed to cover the light diffuser sheet 16. Also, where appropriate, a polarized reflection plate (not shown) may be disposed to cover the light diffuser sheet 16, and a light reflector sheet (not shown) may be disposed at a major surface of the light conductor plate 12 opposite to the light outlet surface 12b. And, an aluminum case 20 is placed over the major surface of the light conductor plate 12 opposite to the light outlet surface 12b, while a frame-like metal case 22 is placed over the prism sheet 18 (toward the light outlet surface 12b).
The aforementioned optical sheets, specifically the light diffuser sheet 16, the prism sheet 18, and the polarized reflection sheet (not shown) are provided for the purpose of enhancing and uniforming the brightness and adjusting the viewing angle at the light outlet surface 12b of the light conductor plate 12 of the spread illuminating apparatus 10 (refer to, for example, Japanese Patent Applications Laid-Open Nos. 2002-343122 and 2001-43721), and the area size of the optical sheets is determined to be smaller than the area of the light outlet surface 12b of the light conductor plate 12 to just cover the effective surface area of the light conductor plate 12 contributing to illumination and at the same time determined to be slightly larger than the window area of the frame-like metal case 22 thereby prohibiting the edge portions thereof from being visible.
It has been demanded for the spread illuminating apparatus to be maximally reduced in size and to achieve the highest possible brightness. Under such the circumstances, in order to increase the effective surface area contributing to illumination without increasing the entire dimension of the spread illuminating apparatus 10 (that is to say, in order to reduce the non-effective area), a rim portion 22a of the frame-like metal case 22 is desired to be narrowed. The rim portion 22a, when narrowed, leads to a decreased overlap margin with the optical sheets (the light diffuser sheet 16, the prism sheet 18, and the like), and it is possible that when the optical sheet shrinks due to heat, the rim edge of the optical sheet is visible raising an aesthetic defect. Consequently, it is essential that the optical sheets, especially the light diffuser sheet 16 disposed closest to the LEDs 14 emitting heat, be sized and attached with a great care when the rim portion 22a of the frame-like metal case 22 is narrowed.
Also, when the rim portion 22a of the frame-like metal case 22 is narrowed, the distance is decreased between the LED 14 and the window edge of the frame-like metal case 22 thus constituting a disadvantage in the effort to handle the problem that the brightness is notably higher in front of the LED 14 (hot spot), and further stringent countermeasures are required to deal with the hot spot problem.